As a hydraulic drive unit arranged on a construction machine to perform a combined operation of plural hydraulic cylinders, the hydraulic drive unit disclosed in JP-A-2000-337307 is known, for example. This hydraulic drive unit is arranged on a hydraulic excavator. FIG. 4 is a hydraulic circuit diagram showing an essential construction of the hydraulic drive unit disclosed in JP-A-2000-337307, and FIG. 5 is a side view of the hydraulic excavator on which the hydraulic drive unit shown in FIG. 4 is arranged.
The hydraulic excavator shown in FIG. 5 is provided with a travel base 1, a revolving upperstructure 2 arrange on the travel base 1, a boom 3 mounted pivotally in a vertical direction on the revolving upperstructure 2, an arm 4 mounted pivotally in a vertical direction on the boom 3, and a bucket 5 mounted pivotally in a vertical direction on the arm 4. The boom 3, arm 4 and bucket 5 make up a front attachment. Also provided are a boom cylinder 6 for driving the boom 3, for example, as a first hydraulic cylinder; an arm cylinder 7 for driving the arm 4, for example, as a second hydraulic cylinder; and a bucket cylinder 8 for driving the bucket 5.
FIG. 4 illustrates the hydraulic drive unit arranged on the above-mentioned hydraulic excavator and provided with directional control valves of the center bypass type for driving the boom cylinder 6 and the arm cylinder 7, respectively, in the hydraulic drive unit.
As illustrated in FIG. 4, The boom cylinder 6 is provided with a bottom chamber 6a and a rod chamber 6b. When pressure oil is fed to the bottom chamber 6a, the boom cylinder 6 is caused to extend so that boom raising is performed. When pressure oil is fed to the rod chamber 6b, the boom cylinder 6 is caused to contract so that boom lowering is performed. The arm cylinder 7 is also provided with a bottom chamber 7a and a rod chamber 7b. When pressure oil is fed to the bottom chamber 7a, arm crowding is performed. When pressure oil is fed to the rod chamber 7b, arm dumping is performed.
The hydraulic drive unit, which includes the boom cylinder 6 and arm cylinder 7 as mentioned above, is provided with an engine 20, a main hydraulic pump 21 driven by the engine 20, a boom-controlling directional control valve 23 as a first directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump 21 to the boom cylinder 6, an arm-controlling directional control valve 24 as a second directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump 21 to the arm cylinder 7, a boom control device 25 as a first control device for performing remote switching control of the boom-controlling directional control valve 23, an arm control device 26 as a second control device for performing remote switching control of the arm-controlling directional control valve 24, and a pilot pump 22 driven by the engine 20.
The boom-controlling directional control valve 23 is arranged on a line 28 extending to a delivery line of the main hydraulic limp 21, whereas the arm-controlling directional control valve 24 is arranged on a line 27 extending to the above-mentioned delivery line.
The boom-controlling directional control valve 23 and the bottom chamber 6a of the boom cylinder 6 are connected with each other via a main line 29a, and the boom-controlling directional control valve 23 and the rod chamber 6b of the boom cylinder 6 are connected with each other via a main line 29b. Similarly, the arm-controlling directional control valve 24 and the bottom chamber 7a of the arm cylinder 7 are connected with each other via a main line 30a, and the arm-controlling directional control valve 24 and the rod chamber 7b of the arm cylinder 7 are connected with each other via a main line 30b. 
The boom control device 25 is connected to the pilot pump 22. A pilot pressure produced responsive to a manipulation of the boom control device is fed via one of pilot lines 25a,25b to its corresponding control chamber of the boom-controlling directional control valve 23, and changes over the boom-controlling directional control valve 23 to a left position or right position in FIG. 4. Similarly, the arm control device 26 is connected to the pilot pump 22. A pilot pressure produced responsive to a manipulation of the arm control device is fed via one of pilot lines 26a,26b to its corresponding control chamber of the arm-controlling directional control valve 24, and changes over the arm-controlling directional control valve 24 to a left position or right position in FIG. 4.
In the hydraulic excavator provided with the hydraulic drive unit constructed as described above, the boom control device 25 shown in FIG. 4 is manipulated upon digging or otherwise handling earth or sand. As a result, a pilot pressure is produced, for example, in the pilot line 25a so that the boom-controlling directional control valve 23 is changed over to the left position in FIG. 4. As a consequence, the pressure oil delivered from the main hydraulic pump 21 is fed to the bottom chamber 6a of the boom cylinder 6 via the line 28, boom-controlling directional control valve 23 and main line 29a, and the pressure oil in the rod chamber 6b is caused to return to a reservoir 43 via the main line 29b and boom-controlling directional control valve 23. As a result, the boom cylinder 6 extends as indicated by an arrow 13 in FIG. 5 so that the boom 3 is caused to pivot as indicated by an arrow 12 in FIG. 5 and boom raising is hence performed.
Concurrently with the above boom-raising manipulation, the arm control device 26 is manipulated and hence, a pilot pressure is produced, for example, in the pilot line 26a so that the arm-controlling directional control valve 24 is changed over to the left position in FIG. 4. As a consequence, the pressure oil delivered from the main hydraulic pump 21 is fed to the bottom chamber 7a of the arm cylinder 7 via the line 27, arm-controlling directional control valve 24 and main line 30a, and the pressure oil in the rod chamber 7b is caused to return to the reservoir 43 via the main line 30b and arm-controlling directional control valve 24. As a result, the arm cylinder 7 is caused to extend as indicated by an arrow 9 in FIG. 5 so that the arm 4 is caused to pivot as indicated by an arrow 11 in FIG. 5 and an arm-crowding operation is hence performed.
In addition to such boom-raising and arm-crowding operations as mentioned above, an unillustrated bucket control device is also manipulated to change over a bucket-controlling directional control valve such that the bucket cylinder 8 shown in FIG. 5 is caused to extend in the direction of an arrow 10 in FIG. 5. As a result, the bucket 5 is caused to pivot in the direction of the arrow 11 such that the desired digging or like work of earth or sand is performed.
FIG. 6 shows characteristic diagrams of pilot pressure characteristics and cylinder pressure characteristics in the above-mentioned combined operation. In the lower diagram in FIG. 6, times of digging work are plotted along the abscissa whereas pilot pressures produced by the control device are plotted along the ordinate. Designated at numeral 31 in the lower diagram in FIG. 6 are pilot pressures produced by the arm control device 26 shown in FIG. 4 and to be fed to the pilot line 26a, in other words, pilot pressures upon crowding the arm whereas indicated at numeral 32 in the lower diagram in FIG. 6 are pilot pressure produced by the boom control device 25 shown in FIG. 4 and to be fed to the pilot line 25a, in other words, pilot pressures upon raising the boom. Signs T1, T2 and T3 indicate time points at which boom-raising operations were performed.
In the upper diagram in FIG. 6, times of digging work are plotted along the abscissa whereas load pressures produced in hydraulic cylinders 6,7, in other words, cylinder pressures are plotted along the ordinate. Designated at numeral 33 in the upper diagram in FIG. 6 are bottom pressures produced in the bottom chamber 7a of the arm cylinder 7, in other words, arm cylinder bottom pressures whereas indicated at numeral 34 are rod pressures produced in the rod chamber 6b of the boom cylinder 6, in other words, boom cylinder rod pressures. When such a combined, boom-raising and arm-crowding operation is performed, a force in the direction of the arrow 12 in FIG. 5 is transmitted to the boom 3 by a counterforce occurred upon digging earth or sand by the bucket 5. Accordingly, the boom cylinder 6 tends to be pulled in the direction of the arrow 13 in FIG. 5, and as a consequence, a high pressure is produced in the rod chamber 6b of the boom cylinder 6 as indicated by the boom rod pressure 34 in the upper diagram in FIG. 6.
In the above-mentioned conventional technology, the digging or like work of earth or sand can be performed without an inconvenience or problem through a combined, boom-raising and arm-crowding operation. When the digging of earth or sand is performed, for example, by an arm-crowding single operation, however, an inconvenience or problem may arise as will be described next.
Specifically, when an arm-crowding single operation is performed where the ground is very hard or under such a situation that a large rock exists in the earth, the pressure in the bottom chamber 7a of the arm cylinder 7 becomes very high. As the boom 3 is pivotally connected to the revolving upperstructure 2, on the other hand, a digging counterforce by the arm 4 is applied in the direction indicated by the arrow 12 in FIG. 5 so that a tensile force is applied in the direction of the arrow 13 to the boom cylinder 6. In this state, the boom-controlling directional control valve 23 is in such a position as closing the circuit. The pressure oil in the rod chamber 6b of the boom cylinder 6, therefore, has no place to which it can drain back, so that the pressure becomes very high. In other words, despite the single arm-crowding operation, the pressure in the rod chamber 6 of the boom cylinder 6 becomes very high, so that a digging counterforce by the arm 4 may not be fully supported by the boom cylinder 6 and the body of the hydraulic excavator may be lifted as illustrated in FIG. 7. Such a situation is unpleasant for the operator, and may become a cause of a reduction in the efficiency of work.
To such a problem, a hydraulic circuit has been proposed in which as illustrated in FIG. 8, for example, a line is arranged to connect the rod chamber 6b of the boom cylinder 6 as the first hydraulic cylinder and the reservoir 43 with each other and an overload relief valve 80 is arranged on the line to release the pressure oil to the reservoir 43 when the pressure rises to at least a predetermined pressure. Such a circuit is, however, extremely disadvantageous from the standpoint of energy efficiency because a great deal of heat is produced in the hydraulic circuit upon occurrence of a situation that the overload relief valve 80 continues to release.
With the above-mentioned circumstances of the conventional technologies in view, the present invention has as an object the provision of a hydraulic drive unit which, upon performing an operation that pressure oil is fed to the bottom chamber of the second hydraulic cylinder, an excessive pressure can be released from the rod chamber of the first cylinder and the pressure oil in the rod chamber can be effectively used.